1. Field of the Invention
The present invention relates to a canister which serves to prevent fuel vapor, generated in the fuel tank of a motor vehicle, from diffusing into the atmosphere. More particularly, it relates to a canister which prevents fuel vapor from diffusing into the atmosphere even in a case where the fuel vapor has increased in excess of the adsorbing capability of the canister during the running of a motor vehicle.
2. Description of the Prior Art
In general, fuel for use in a motor vehicle is highly volatile, so that it vaporizes in a fuel tank under heat from the engine and other parts of the vehicle during the drive of the vehicle or the stop thereof after the drive. When the fuel vapor has diffused into the atmosphere, the atmospheric air is polluted with hydrocarbons contained in the fuel. Therefore, a canister as shown in FIG. 3 has heretofore been used for preventing the diffusion of the fuel vapor into the atmosphere and for recycling the vaporized fuel.
With this prior-art technique, fuel vapor, generated in a fuel tank 3 when the drive of a motor vehicle has stopped, passes through an air breather pipe 8 and then through an adsorbent 2 contained in a canister 1 and comes into communication with the atmosphere owing to a vent pipe 15. On this occasion, the fuel component of the fuel vapor is absorbed to the adsorbent 2, and only purified air is emitted into the atmosphere.
Besides, while the vehicle is being driven, a negative pressure is developed in the suction system 4 of an engine by the opening of a throttle valve 5, and the fuel adsorbed to the adsorbent 2 is vaporized and is recycled into the suction system 4 through a suction pipe 9 by the negative pressure, to become part of the fuel of the vehicle. Midway of the air breather pipe 8, there are interposed in parallel a first check valve 13a which is opened when the pressure of the upper space of the fuel tank 3 has risen to a predetermined value, thereby to release the pressure, and a second check valve 13b which is opened when the pressure of the upper space has lowered to a predetermined value, thereby to introduce the atmospheric pressure. It is also known that, in a case where the fuel has a low vapor pressure and vaporizes slightly, the first check valve 13a cuts off the air breather pipe 8, thereby functioning to prevent the fuel vapor from being wastefully adsorbed to the adsorbent 2 (refer to the official gazette of Japanese Patent Application Laid-open No. 53451/1986).
With the prior-art technique, in principle, the fuel vapor is adsorbed at the stop of the drive of the motor vehicle and is recycled during the drive. The fuel vaporizes during the drive more than at the stop of the drive on account of the influences of heat and vibrations. During the drive, however, the fuel is recycled through the suction pipe 9, with the result that it seems to be recycled entirely.
In recent years, however, vehicular engines have become high in performance and high in power owing to the addition of a turbosupercharger, the adoption of a fuel injection system based on an electronic control, etc., whereas the ambient temperatures of the engines have heightened. Therefore, the temperature of the fuel during the drive tends to rise, and the amount of vaporization of the fuel increases. Especially in case of high-speed running, there arises a situation where the check valve 13a is almost held open, so the fuel vapor cannot be entirely recycled, and where the adsorbing capability of the adsorbent 2 is exceeded, so the fuel vapor is emitted into the atmosphere.
In order to improve this situation, it is thought out that the recycling capability of the canister is enlarged by thickening the suction pipe 9. However, when the suction pipe 9 is thickened, inconveniently the mixture of the engine becomes lean during the idling of the engine and the low-speed running of the vehicle. Moreover, enlargement in the capacity of the canister is spatially disadvantageous and is costly. Another problem is that the adsorption of the fuel cannot be continued when the high-speed running has been continued for a long time.